The present invention relates in general to telecommunication systems, and is particularly directed to a new and improved signaling arrangement and method for supplying emergency xe2x80x98lifelinexe2x80x99 plain old telephone service (POTS) connectivity over a two-wire metallic digital subscriber loop (DSL) pair, such as that employed for ISDN communications. A (quasi) ground fault interruption (GFI) signal is generated by a remote terminal (RT) in response to a customer""s POTS phone going off-hook, during a lack of normal operating condition of the DSL circuit. This GFI signal is used to activate a POTS by-pass path in the RT and a POTS by-pass path at a central office terminal (COT).
Digital telecommunication systems, such as, but not limited to integrated services digital network (ISDN) communication systems, enable telecommunication service providers to supply multiple types of signalling channels from a central office site using a local twisted pair, termed a digital subscriber loop (DSL), to a network termination interface at a customer premises site. FIG. 1 diagrammatically illustrates a reduced complexity example of a span-powered DSL system for supplying POTS and ISDN communication services over a local loop. As a non-limiting example, such a span-powered DSL system may be of the type described in the U.S. Pat. No. 5,774,316, to J. McGray et al, entitled: xe2x80x9cGround Fault Detector for Line-Powered Telephone Network,xe2x80x9d assigned to the assignee of the present application and the disclosure of which is incorporated herein.
As shown in FIG. 1, the span-powered DSL system comprises a central office site 1, located at a xe2x80x98westxe2x80x99 end of a twisted pair 30, and a customer premises site 2 that terminates the xe2x80x98eastxe2x80x99 end of the loop. The central office site 1 includes a conventional POTS switch 3 (such as a 5ESS switch manufactured by ATandT), which: contains a plurality of line termination circuits (or line cards) 4, and an ISDN switch 5, which contains a plurality of ISDN cards 6. These respective circuits are interfaced with the local loop 30 by means of multiplexer circuitry 7 within a central office terminal (COT) 10. Power for the COT 10 is provided by way of local isolated span supply 8, which is (transformer) coupled to the metallic link from the central office. At the customer premises site 2, the local loop 30 is coupled directly to a remote terminal 20, which serves a customer""s digital terminal equipment (DTE) 24 and local POTS phone 29. As in the central office terminal, these circuits are interfaced with local loop 30 by means of multiplexer circuitry 23; power for components of the remote terminal is provided by way of a local span supply 28, which is coupled to wireline link 30.
Although the public service commissions of local jurisdictions, where customer premises equipment is installed, require telephone service providers (regional Bell operating companies) to ensure that each customer is continuously provided with emergency (911) telephone service in the event the DSL equipment at the remote terminal becomes non-functional (such as may be due to a loss in synchronization or a less than fully synchronized condition of the digital circuitry), the service provider is not permitted to dictate that a subscriber install a particular piece of equipment.
As a consequence, service providers customarily require that their residential subscribers maintain a separate POTS line as an emergency xe2x80x98lifelinexe2x80x99 adjunct to the DSL (ISDN) service, so that the residential customer is, in effect, forced to subscribe to an additional form of xe2x80x98fire insurancexe2x80x99. Namely, the extra POTS line (which normally goes unused) is intended to prevent residential customers from losing access to telephone service, particularly in the event of an emergency, where expedient 911 service may be critical. Such a requirement obviously constitutes a substantial cost penalty to the customer, who is faced with both a potential installation fee, and an unwanted monthly bill for a normally unused auxiliary line.
In accordance with the present invention, the need to ensure continuous telephone service to a residential customer premises served by a DSL, such as one used for ISDN, without requiring that the customer pay for an additional (and potentially unnecessary) POTS line as an emergency back-up, is successfully addressed by a modification of conventional signaling circuits and network termination interface components of the central office terminal and the remote terminal to include an auxiliary POTS by-pass path. Each auxiliary path is normally decoupled from its terminal""s signaling equipment, but is controllably coupled in circuit with the wireline pair, so as to provide an emergency or xe2x80x98lifelinexe2x80x99 POTS by-pass path between the central office and the subscriber""s POTS phone in the event that the subscriber needs to place a POTS call during a less than fully functional condition of the DSL circuitry.
In particular, the remote terminal circuitry serving the customer premises site is augmented to generate a (quasi) GFI signal should two conditions occur. The first is a lack of normal operation of the DSL circuit, as may result from a lack of digital link synchronization, as a non-limiting example. The second condition is a POTS phone at the remote terminal going off-hook. Unless both of these conditions occur, signaling path connectivity is provided through the central office terminal and the remote terminal through DSL/POTS multiplexer circuitry and a pair of energized auxiliary relay circuits.
This normal operation connectivity state continues to be maintained, as long as such normal DSL operating conditions are present, so as to provide both analog and digital communications, including allowing customer to place a POTS call, by way of the fully operational DSL circuitry of each of the COT and the RT. This normal operation connectivity state will also be maintained in the absence of a fully functional DSL link, so long as there is no attempt by a customer to place a POTS call. While the absence of a fully functional DSL link could be due to an operational anomaly, it is also an inherent condition of the DSL link during initialization (including transmission and detection of a training sequence) of the DSL circuit.
If the customer attempts to place a POTS call (goes off-hook) during a less than fully functional state of the DSL circuit, the state of a DSL sync input to a POTS by-pass condition detector in the remote terminal will indicate non-proper operation of the digital circuit; in addition, the off-hook input to the POTS by-pass detector will be asserted. As a consequence, a (quasi) ground fault interrupt signal generation circuit will couple a (quasi) GFI signal to an RT power supply and relay control circuitry in the remote terminal, and al so over the wireline DSL path to GFI sense circuitry within the central office terminal.
In response to the GFI signal, control circuits in each of the COT and the RT de-energize relay circuits associated with respective POTS by-pass paths around the DSL multiplexers, and thereby provide a direct analog (POTS) by-pass signaling path between the customer""s POTS phone at the RT and a POTS switch at the COT. Once this POTS by-pass signaling path has been established between the customer""s POTS phone and the POTS switch, an on-hook detector in the central office terminal monitors the state of the by-pass link in order to determine when the call has ended (the customer has gone back on-hook).
When the POTS call has been terminated (the customer""s phone goes back on-hook), the on-hook detector signals the COT""s relay control circuit, causing the COT""s relay coil to be re-energized, thereby returning the relay switch condition in the COT to its energized state for normal DSL operation conditions and decoupling the POTS by-pass path in the COT. This relay switching operation in the COT also causes span power to be reapplied to the DSL link, re-establishing power in the RT. With span-sourced power restored in the RT, its relay control circuit re-energizes RT relay coils for the POTS by-pass path, decoupling the POTS by-pass path in the RT, and restoring the RT to normal DSL communications.